Preparation of thoriated nickel-chromium alloy powder

ABSTRACT

Nickel-chromium alloy powder containing dispersed refractory oxide particles are produced by heating nickel powder particles containing physically inseparable sub-micron refractory oxide particles, a major portion of which are fixed in the surfaces of the nickel particles, in contact with a chromium source material, such as chromium powder or chromium oxide, in a flowing atmosphere of pure dry hydrogen at an elevated temperature and for a time sufficient to effect diffusion of chromium into the nickel particles and to reduce excess oxygen to an acceptable level.

.1111 :1,/ oa,oo7 Oct. 16, 1973 1 mementos or mom/1mm mcrtnL-cnnoMwM ALLOY POWDER [75] inventors: David John Ivor Evans, North Edmonton, Alberta; David Alan Wayne Fustukian, Edmonton, Alberta; Leon Frederick Norris;

Robert Wiiiiam Fraser, both of For:

Saskatchewan, Alberta, all of Canada 1 [73] Assignee: Sherritt Gordon Mines Limited,

Toronto, Ontario, Canada UNiTED STATESv PATENTS 3,469,967 9ll969 Meddings et al 75/0.5 AC

3,526,498 Evans et aL- 75/05 AC.

9/1970 3,317,285. 5/1967 3,382,051 5/1968 3,388,010 6/1968 3,393,067 7/1968 1 3,446,679 5/1969 Marsh l48/lL5 F Primary Examiner-W. W. Stallard Atzorney -Frank I. Piper et al.

571 ABSTRACT Nickel-chromium alloy powder containing dispersed refractory oxide particles are produced by heating nickel powder particles containing physically inseparable sub-micron refractory oxide particles, a major portion of which are fixed in the surfaces of the nickel particles, in contact with a chromium source material, such as chromium powder or chromium oxide, in a flowing atmosphere o pure dry hydrogen at an el e vated temperature and for a time sufficient to effect diffusion of chromium into the nickel particles and to reduce excess oxygen to an acceptable level,

4 Ciaims, 2 Drawing Figures PATENTED OCT 1 6 I975 FIG. 1

FIG. 2-

PREPARATION OF THORIATED NICKEL-CHROMIUM ALLOY POWDER This is a Divisional application of application Ser. No. 813,214 filed Apr. 3, 1969 now U.S. Pat. No. 3,716,357.

This invention relates to nickel-chromium alloy powders containing dispersed refractory oxide particles and to a process for producing such powders. More particularly, it is concerned with such powders and their production in a form which is particularly adapted for direct fabrication into wrought disperson strengthened nickel-chromium and nickel-chromium base alloys by powder metallurgy techniques.

Advances in the design of turbo-jet engines and the advent of aerospace vehicles have resulted in the need for materials which can withstand high temperatures for extended periods. Although improvements in super alloys and in particular nickel base super alloys have greatly extended the high temperature capabilities of these materials, the gap between the requirements of the design engineer and the materials commercially available has not narrowed appreciably. One new group of materials which appears to have considerable potential for application in the temperature range of l,800 to 2,400F. are the so-called dispersion strengthened or dispersion hardened nickel and nickel base alloys.

in these materials, the high temperature strength Y properties are considerably improved by the presence in the metal or metal alloy matrix of the material of uniformly dispersed, ultrafme refractory particles which are stable and substantially insoluble in the matrix at elevated temperatures. These materials are generally fabricated by powder metallurgical methods involving the compacting and sintering of metal powder mixtures containing the desired metal and refractory oxide ingredients, followed by mechanical working of the sinter body to produce a fully dense, wrought shape. Optimum refractory oxide dispersion and maximum strength properties are obtained by the use of power compositions in which the refractory oxide constituent is in the form of discrete particles incorporated inv one or more of the metal powder constituents of the mixture such that they are mechanically inseparable therefrom.

Serious difficulties are encountered in applying the conventional powder metallurgy techniques to the fabderived from alloying chromium with nickel is not fully realized. Also, chromium oxide contaminates adversely affect the oxidation resistance of the wrought product and are apparently a factor in causing undesirable agglomeration of the refractory oxide particles during processing, which further adversely affects the high temperature service characteristics of the alloy product. Further, the presence of chromium oxide contamijoining of the wrought material in that the oxides tend to react with the brazing alloy causing gas holes which weaken the brazed joint.

These problems can be obviated to some extent by providing a small amount of a getter material, such as magnesium, in the nickel-chromium-refractory oxide powder mixture, by taking precautions to employ chromium powders with an extremely low oxide content or by sintering the billets in a very dry hydrogen atmosphere for extended periods. However, these methods are generally unsatisfactory for a commercial scale operation. Also, even in absence of chromium oxide contaminants, with the conventional powder mixing approach there is a tendency for undesired refractory oxide stringers to form at the interfaces between the nickel and chromium powder particles during fabrication. i Y

An obvious and expedient solution to all these problems is to employ a chromium oxide-free, pre-alloyed nickel-chromium powder containing uniformly dispersed, physically inseparable refractory oxide particles. However, heretofor, no economic, efficient method for producing such powders was known. One known method, described in U.S. Pat. No. 2,972,529,

involves the coprecipitation of preformed, sub-micron refractory oxide particles together with the hydrous oxides of nickel and chromium, followed by filtering, I

washing, drying, pulverizingand reducing the coprecipitate at elevated temperature with dry hydrogen.

Although this process may becapable of producing a in the steps of washing and drying the coprecipitate and drying the hydrogen as well as high capital investment in equipment for carrying out these operations. The result is that the final product is very costly and its use is necesssarily restricted to applications where material costs are not an important consideration.

, A principal object of the present invention, therefore, is to provide a simple, efficient and economic pro-' cess for the production of nickelchromium alloyrefractory oxide powder compositions suitable forpowder metallurgical fabrication of dispersion strength-' ened nickel-chromium and nickel-chromium base alloy products. A further object of this-invention is the provision of a novel form of pre-alloyed nickel-chromium alloy powder containing mechanically inseparable submicron refractory oxide particles in integral association therewith. 7

These and other objects of this invention are accomplished by means of a surprisingly simple process involving the steps of providing nickel powder particles containing physically inseparable, sub-micron refractory oxide particles a major portion of which are fixed in the surfaces of said nickel particles; mixing the nickel-refractory oxide particles with a finely divided chromium source material crnsisting of chromium metal,

' chromium oxide, chromium hydroxide or mixtures thereof in an amount sufficient to provide from about 10 to about 35 wt. percent chromium in the powder mixture; forming a stationary bed of the resulting powder mixture and heating said bed at a temperature above about l,200F. but below the sintering temperature of the powder mixture in a flowing atmosphere of pure dry hydrogen; continuing the heating for a time sufficient to effect diffusion of substantially all available chromium into the nickel particles and to reduce the amount of oxygen in the powder mixture to less than 0.6 oxygen in excess of the oxygen contained in the refractory oxide constituent; and recovering the resulting refractory oxide containing nickel-chromium alloy powder product.

Substantially complete alloying of nickel and chromium and removal of excess oxygen generally can be obtained by heating within the aforesaid temperature range in flowing hydrogen having a dew point below about 4F. for from about 1 to about 100 hours with the actual time depending on the sintering temperature and the hydrogen dew point employed. This result is surprising in that the literature indicates that only about 80 percent reduction of pure Cr o should be obtainable under these conditions. The increase in the remanner of preparation of the nickel-refractory oxide power starting material isnot important. However,

since the characteristics of the nickel-refractory oxide powder are directly reflected in the product, it is desirable that the refractory oxide constituent be in discrete,

duction reaction rate and efficiency apparently results from the fact the nickel in the powder mixture acts as a sink for free chromium during the Cr O reduction reaction and thus changes the equilibrium of the reaction.

Chromizing of the nickel-refractory oxide starting material in accordance with the invention takes place more rapidly as the reaction temperature is increased. However, for any given nickel-thoria powder, there is an upper limit on the chromizing temperature above which excessive sintering occurs with the result that the product is not a free-flowing powder. We have found that for nickel-refractory oxide powders having a major portion of the refractory oxide particles fixed in the surfaces thereof, the maximum permissible chromizing temperature for production of a free-flowing powder product is a function of the volume loading of the refractory oxide in the nickel powder. For nickelrefractory oxide powders containing up to volume percent refractory oxide, the relation may be expressed by the equation:

where Tom is the maximum chromizing temperature in The particles of the nickel-chromium alloyrefractory oxide powder product of the invention retain v essentially the same physical characteristics as the nickel-refractory oxide starting particles. That is, the particle size and shape, the refractory oxide size and the refractory oxide distribution of the nickelrefractory oxide starting material determines what these characteristics are in the nickel-chromium alloyrefractory oxide powder product. Thus, the quality and character of the alloy powder product is controlled primarily by controlling the quality and character of the starting nickel-refractory oxide powder.

A preferred novel product of the process is nickelchromium alloy-refractory oxide powder containing about 10 wt. percent to about 35 wt. percent chromium, about 0.5 to about 20 percent by volume submicron refractory oxide and the balance nickel and consisting essentially of grape-like clusters, up to about 50 microns in size, of generally spheroidal nickelsub-micron sized form uniformly distributed throughout the nickel powder and that there be essentially no large volume of nickel devoid of refractory oxide particles. Since it is essential that the major portion of the refractory oxide be fixed on the surfaces of the nickei particles this latter requirement can best be met by using nickel-refractory oxide powder comprised of clusters of very fine particles of nickel, 0.5 micron in size or smaller, having refractory oxide particles fixed in their surfaces.

Co-pending United States applications Ser. Nos.

543,495 and 604,129 now U.S. Pat. Nos. 3,469,967 and 3,526,498 respectively describe composite nickelthoria powders which are particularly suitable for use in the process of this invention. These powders, which are produced by methods involving direct hydrogen reduction of basic nickel compounds in aqueous suspensions, consist of irregular-shaped particles of nickel comprised of clusters of sub-particles of nickel between about 0.2 and about 0.5 micron in size. The nickel subparticles have ultra-fine thoria fixed in their surfaces and may occur singly or be agglomerated in grape-like clusters up to 50 microns or more in size. The thoria particles preferably are between 2 and 50 millirnicrons in size and are uniformly distributed on the surfaces of the nickel-sub-particles. Typical powders have an apparent density between 0.5 and 2.5 grams per cubic centimetre, a Fisher number of less than 2.0 and preferably contain from about 2.0 to about 4.0 percent by volume of one or morerefractory oxides. The refractory oxide particles must have a melting point higher than the matrix metal, a good thermal stability, low solubility in the matrix metal and should be non-reactive with the matrix metal at elevated temperatures in the 1 order of about 2,400F. There are a large number of refractory oxides which satisfy the conditions necessary for use as a dispersed phase. For example, yttria, ceria and thoria have allbeen shown to be particularly suitable. Because of its ready commercial availability and high free energy of formation value, thoria is a preferred dispersoid.

nickel powder in the form of discrete particles, e.g. by-

the addition ofthoria powder or sol or thorium nitrate can be mixed with the nickel powder and thencalcined to form thoria in situ. Nickel refractory oxide powders can be produced by the latter procedure which have characteristics similar to the powders produced by the direct hydrogen reduction of refractory oxide impregnated basic nickel carbonate in that the particles may be very fine, in the order of 2 microns or less, the refractory oxide is mechanically inseparable from the nickel and a substantial portion ofit is fixed on the surfaces of the nickel particles.

The chromium source material employed in the process of the invention may be commercial, high purity chromium powder, oxides of chromium, chromium hydroxide or mixtures of these. Chromium powder is preferred because it blends easily with the nickelrefractory oxide particles and requires less time for reduction of oxides. Chromium powder having a particle size smaller than the particle size of the nickelrefractory oxide powder particles is preferred, although satisfactory results can beobtained with relatively coarse chromium powder having an average particle size up to 50 microns.

According to the invention, the nickel-refractory oxide powder and chromium source powder are thoroughly mixed in any conventional mixing or blending equipment to provide a substantially homogeneous powder mixture. The relative proportions of each ingredient will, of course, depend on the type of chromium source material employed and the chromium content desired in the nickel-chromium alloy product.

is close to the solubility limit of chromium in nickel.

The powder mixture containing the appropriate quantity of chromium or chromium compound is formed into a bed in a suitable heat resistant container which will enable the mixture to be heated in contact with a flowing dry hydrogen atmosphere. For example, shallow, elongated open boats or trays are suitable for this purpose.

The container with the powder bed is inserted into a furnace such as an indirectly heated tube furnace and is heated in a flowing atmosphere of dry hydrogen at a temperature above about 1,200F. but below the powder mixture sintering temperature. This may be done on a batch basis or on a continuous basis, e.g., by placing the trays on a conveyor, such as a belt, which moves continuously through the furnace. Heating in flowing dry hydrogen is continued for a time sufficient to cause the chromium content of the powder mixture to diffuse into the nickel-refractory oxide powder and to reduce the amount of oxygen in excess of that in the refractory oxide to below about 0.6 wt. percent. The chromizing rate is not affected by the depth of the powder mixture bed or the rate of hydrogen flow, as long as sufficient hydrogen is available to reduce the chromium oxides in the powder mixture. Also, the hydrogen contacting the powder mixture must be pure and very dry, having a dew point below about 4F. and preferably between about -40F. and l50 F.

The actual time required for complete chromizing in any given case will depend on the chromizing temperature, the hydrogen dew point and the nature and size of the chromium source material. The chromizing rate I nickel-refractory oxide powders containing up to 25 volume percent refractory oxide, the relation may be expressed by the equation:

where Tcm is the maximum chromizing temperature in K, Tm is the melting point of the final nickelchromium alloy composition andfis the volume frac tion of refractory oxide in the nickel-refractory oxide powder. In some cases, where chromizing temperatures I ume percent thoria, the preferred chromizing tempera- I near the upper limit as determined from the foregoing equation are used, the alloy powder product may be slightly caked but it is readily comminuted to a freeflowing powder by light grinding. Preferably the chromizing operation is carried out at temperatures 50 to 100 F. degrees below the maximum as determined from the foregoing equation. For example, in the case of nickel-refractory oxide powder containing about 3 volture range is about 1,700F. to l,950F.

For any given temperature, the oxide reduction reaction is much slower than the chromium diffusion reaction for chromium oxides, chromium hydroxide and very fine chromium powder, i.e., powder having a particle size below about 2 microns. This is illustrated in" the'following Table I.

TABLE I v 7 Time Required to Prepare Ni/Cr/ThO Powder at l90OF.

Reaction Time for Reaction at I900F.

Chromium Source Fine Cr Medium Cr Coarse Cr C130, Powder Powder Powder (F.N.=.9 (F.N.=8) '(F.N.=44) Oxides Reduction Reaction (to 0.30 50 35 25 ,64 wt. 51 0,) chromizing Reaction (to l9 wt. Cr) 6 50 20 nace, cooled in air and, if necessary, lightly commi-' nuted to break up caked particles into free-flowing powder. Typical Ni-Cr-refractory oxide powders produced by this process arecharacterized by a Fisher number in the range of 1.0 to 3.5 and apparent density in the range of 1.0 2.0.

FIGS. 1 and 2 of the drawing, which are electron micrographs of a nickel-thoria particle and a nickelof clusters up to about 5.0 microns or more in size of generally spheroidal particles up to about 0.5 micron in size which have sub-micron refractory oxide particles fixed in their surfaces. In the nickel-chromium alloy powder of the invention, the spheroidal particles contain from about to about 35 wt. percent preferably about wt. percent chromium, uniformly alloyed with nickel. in the preferred powders, the uniformly alloyed, spheroidal nickel chromium particles have a diameter between about 0.2 and about 0.5 micron and contain up to about volume percent, preferably 0.5 to about 6.0 volume percent, of discrete refractory oxide particles between about 2 and about 50 millimicrons, preferably between 2 and millimicrons in size, the major portion of which is fixed in and uniformly distributed over their surfaces. The oxygen content of the powder, exclusive of oxygen combined in the refractory oxide is below about 0.6 wt. percent and preferably below 0.01 wt. percent.

The Ni-cr refractory oxide powders of this invention are particularly useful for fabrication of dispersion strengthened wrought nickel-chromium or nickel chromium base alloy products by powder metallurgy techniques. The powder used alone or combined with one or more alloying metals such as cobalt, molybdenum,

tungsten, copper, aluminum, titanium, etc., enables the production of wrought products having greatly improved high temperature service characteristics.

The process and product of the invention is further illustrated by the following examples:

EXAMPLE 1 Nickel-thoria powder prepared in accordance with the process of Canadian Pat. No. 786,268 was deoxidized by heating at 1,500F. for 15 minutes in dry hydrogen. The de-oxidized nickel-thoria powder had The chromium source material was finely divided chromium powder having the following characteristics:

Fisher Number 8.0 0, wt. 95 0.67 N, wt. 0.021 C wt. 0.04 5 wt. 0.03

The de-oxidized nickel thoria powder was mixed with the chromium powder in a high speed blender for 2 minutes to produce a blend containing 78 wt. percent nickel, 19.7 wt. percent chromium and 2.1 wt. percent cooling zone of a tube furnace. The powder blend was contacted with dry hydrogen in the cooling zone for 15 minutes and then inserted into the heating zone where it was heated at 1,900F. in a flowing H atmosphere having a dew point at point of entry of-1'30F. for 52 hours. The boat was removed from the furnace and cooled. The powder product was slightly caked but was easily broken up into a free-flowing powder having a Fisher number of 2.9 and an apparent density of 1.5 gm/cc. The product contained 19.7 percent chromium, 3,000 parts per million 0 (including 2,800 ppm oxygen contained in the thoria), 80 parts per million nitrogen, 125 parts per million carbon' and 35 parts per million sulphur. X-ray diffraction analyses indicated that the chromium was in a uniform solid solution with the nickel. The powder was non-magnetic confirming the X-ray diffraction results. Microscopic examination showed the powder had essentially the same appearance as the nickel-thoria starting material.

A sample of the powder was statically compacted into 60 gram billet measuring 1.25" X 0.2". The billet was sintered in dry hydrogen, hot rolled, annealed and hot rolled a second time to produce a wrought strip.

The strip was prepared into a tensile specimen which has a 2 inch gauge. length and one-half inch gauge width. The specimen was heated to 2,000-"F. in air and tested in tension using a strain rate of 0.025 in/per min.

The results were as follows: UTS 12,400 p.s.i., YS

10,600 p.s.i., elongation 5.

EXAMPLE 2 A sample of the nickel-thoria powder of Example 1 was blended into an aqueous slurry of chromium hydroxide. The resulting mixture was filtered and dried. 300 grams of the mixture were placed into a 2 inch diameter tube which was positioned in an indirectly heated furnace. The charge was purged with purified hydrogen until the dew point was F. The tube was then placed inside the furnace and heated at a temperature of 2,100F. The dew point was measured on the exit side'of the tube. The hydrogen flow through the charge was maintained at 0.2 to 0.6 standard cubic feet per minute. After about 20 hours, the dew point of the, i

' exit gas dropped to below 50F. and the charged tube was removed from the furnace and cooled. The reduced product was slightly sintered but was easily com- 'minuted to a free-flowing powder. No magnetic material was present indicating that the chromium had alloyed with the nickel during the reduction. A sample of the powder was fabricated into a wrought strip and tested as described in Example 1. The UTS at 2,000 F. was 9,100 p.s.i., YS 7,900 P.S.l. and elongation 3.

EXAMPLE 3 A nickel-thoria powder was prepared by adding 6,000 grams of type B carbonyl nickel powder to 4.5 litres of distilled water containing 339 gramsof hydrated thorium nitrate, vigorously stirring, drying and then calcining at 1,400F. for 3 hours in dry hydrogen. The resulting powder which contained 2.5 volume Th0: was blended with chromium powder as in Example l. The mixture was heated at 1,900F. for 15 hours in purified dry hydrogen. The product was a freeflowing powder having 16.8 percent chromium in solid solution with the nickel. A sample fabricated and tested as in Example 1 exhibited a UTS of 7,800 p.s.i. at

2,0OOF.

It will be understood, of course, that modifications can be made in the preferred embodiment of the present invention as described hereinabove without departing from the scope and purview of the appended claims.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. A powder metallurgy composition adapted for use in fabrication of high temperature resistant alloy products comprising a nickel-chromium alloy-refractory oxide powder comprised of clusters up to about 50 microns in size, of generally spheroidal, uniformly alloyed nickel-chromium alloy particles, said spheroidal particles having a diameter up to about 5 microns and consurfaces of said spheroidal alloy particles and said alloy particles containing less than 0.6 percent oxygen in excess of that contained in the refractory oxide constituent.

2. The powder composition of claim 1 wherein said composition contains about percent chromium.

. 3. The powder composition of claim 2 wherein the refractory oxide is thoria having a particle size within the range of 2 to 50 millimicrons.

' 4. The powder composition of claim 3 wherein the thoria comprises from about 0.5 to about 6.0 volume percent of the'composition. 

2. The powder composition of claim 1 wherein said composition contains about 20 percent chromium.
 3. The powder composition of claim 2 wherein the refractory oxide is thoria having a particle size within the range of 2 to 50 millimicrons.
 4. The powder composition of claim 3 wherein the thoria comprises from about 0.5 to about 6.0 volume percent of the composition. 